Maintaining appropriate hydration level is critical for health and performance of humans and other mammals. Water lost through processes including perspiration and respiration must be replaced. Fluid losses of between 2-3% of body mass detrimentally affect cardiovascular function, thermal dissipation, and exercise performance. Overhydration can also detrimentally affect exercise performance (e.g., due to electrolyte imbalance) and stress a subject's kidneys. Relying upon thirst as a feedback mechanism to trigger demand for fluid intake may not be adequate to maintain an optimal hydration level, since a sensation of thirst sufficient to cause a subject to drink may not be triggered until after the subject is already dehydrated.
One method to assess hydration is to periodically weigh a subject under controlled conditions. For example, over a bout of exercise, a reduction in body weight measured nude before and after exercise will indicate a state of dehydration. While nude body weight changes may be used to assess acute changes in hydration status during a single exercise bout, over longer periods of time, body weight changes may be influenced by many factors other than change in hydration status, such as: food intake, bowel movements, and changes in body composition. As a result, measurement of body weight over a prolonged period is an inaccurate way of assessing whole body hydration status. But even in a single bout of exercise, it may be highly impractical to stop and subject for purposes of measuring nude body weight to assess hydration status.
Other known methods to assess hydration status involve use of testing of urine or blood. For example, urine specific gravity is a common standard among certain physicians. For patients that can be monitored over time, total urine output or urine specific gravity may be used as a metric. Hydration status can also be assessed using a blood sample, since an increase in plasma osmolality can often identify a state of dehydration, but such sensing requires invasive collection of a venous blood sample by a qualified phlebotomist. In numerous settings, use of urine or blood for assessment of hydration status can be highly impractical.
For many reasons, saliva is an ideal choice for development of a rapid, point-of-care diagnostic measurement for dehydration and/or stress. The sample is easily obtained with minimal invasiveness. No blood must be drawn. In many cases, it is difficult for an individual or health care provider to access urine in a patient (especially for the elderly or infants). Also, urine assessment could be indicative of a prior state of dehydration because the urine is maintained in the bladder and does not necessarily reflect a subject's current hydration state. It would be desirable to provide a convenient device and method for sensing hydration status without requiring use of conventional laboratory equipment and specially trained personnel.
Detection of a constituent in saliva has been proposed for sensing hydration status, such as in U.S. Patent Application Publication No. 2008/0050451 to Mabry (“Mabry”). Mabry discloses an assay device embodied in a test strip including a series of reaction zones with monoclonal antibodies capable of binding salivary amylase in a saliva sample, and reagents for colorimetric detection of the resulting antibodies with bound salivary amylase to provide a visual determination of salivary amylase concentration as a proxy for hydration status. Unfortunately, variations in biomarker level among different samples due to environmental factors, genetic factors, disease state, or other factors, as well as difficulties in obtaining adequate signal to noise ratio to overcome variations in the assay technique, may reduce the ability of such assays to reliably assess hydration status.
Another approach to sensing hydration utilizing saliva involves sensing the viscosity thereof, such as proposed in U.S. Patent Application Publication No. 2007/0048224 to Howell et al. (“Howell et al.”). Howell et al. proposes use of a sensing element embodied in a water-permeable material (e.g., blotting paper or cloth) or in a tube, and sensing either the extent or duration of migration of saliva through a portion of the sensing element (since migration rate is affected by salivary viscosity), based on the understanding that viscosity of saliva corresponds to hydration status. Unfortunately, various factors can affect the ability of salivary viscosity to predict hydration status. Such factors include recency of food or beverage consumption or use of chewing gum (as recognized by Howell et al. at ¶[0149]), or occurrence of certain disease status affecting salivary viscosity (as recognized by Howell et al. at ¶[0152]). Although Howell et al. propose the use of an environmental temperature sensor, an environmental humidity sensor, a body temperature sensor, a user location sensor, or a user activity level sensor (e.g., a pedometer) to provide further input regarding the optimal amount of fluid to be consumed by a user, Howell et al. do not disclose any means to overcome potential errors in detecting salivary viscosity as a proxy for user hydration status.
Based on the foregoing, the art continues to seek diagnostic devices and methods for sensing hydration state of a mammalian subject that are adapted to overcome one or more of the foregoing limitations.